The present invention relates to a speed control method and system for automotive vehicles, and more particularly to an improvement of a speed control method and system for an automotive vehicle of the type in which the quantity of fuel supply into the engine is controlled in accordance with the actual speed of the vehicle and the rate of change of the actual speed in relation to a desired command speed.
In such a speed control system as described in U.S. Pat. No. 4,250,854 issued on Feb. 7, 1981, a speed-difference calculation circuit is provided to calculate a first time difference between a predetermined period of time corresponding with a desired vehicle speed and the actual period of time corresponding with the actual vehicle speed, and an acceleration calculation circuit is also provided to calculate a second time difference between the preceding and following actual periods of time corresponding with the actual vehicle acceleration. Thus, a correction signal is obtained in dependence on each value of the first and second calculated time differences to control the quantity of fuel supply into the engine so as to maintain the speed of the vehicle at the desired value.
In the speed control system described above, the actual period of time corresponding with the actual speed of the vehicle changes inversely proportional to the actual vehicle speed. This means that resolution or resolving power in calculation of each value of the first and second time differences changes in dependence on change of the vehicle speed and that the correction signal will be erroneously obtained due to change of the resolution in calculation. This results in difficulty of precise control of fuel supply into the engine over a wider range of the vehicle speed.